In recent years, in a grid-tie inverter, which converts, into an AC voltage, a DC voltage supplied from a DC power supply such as a photovoltaic power generation system and a fuel cell, and interconnects the AC voltage to an electric power grid, implementation of higher frequency switching has advanced, and following this, harmonic leakage current and electro-magnetic interference (EMI) have become problems. It is apprehended that the leakage current and the EMI may affect control of the inverter and other instruments, and that an earth leakage circuit breaker may be caused to malfunction. In Japan, a tolerance of the leakage current is prescribed by the Electrical Appliance and Material Safety Law, and the EMI is prescribed by the Voluntary Control Council for Information Technology Equipment (VCCI), and the like. In particular, with regard to the EMI, in recent years, moves toward tightening of regulations therefor have been being accelerated.
In the photovoltaic power generation system, a stray capacitance exists between a solar cell panel and a frame of the solar cell panel, the frame connected to the ground, and it is possible that the stray capacitance can be a route of a high frequency common mode noise. In general, on a surface of the solar cell panel, an insulating layer composed of a glass plate is formed. Such a glass plate has a large plane, and accordingly, when the glass plate gets wet with rain, the stray capacitance between the solar cell panel and the frame is increased, and a high frequency common mode current is also increased. Variations of a high frequency voltage occur in the event where the inverter converts the DC voltage to the AC voltage by switching of a semiconductor element. Therefore, in the inverter, the leakage current and the high frequency noise are inevitable problems.
As general methods for suppressing the leakage current and the high frequency noise, there have been known: a method of isolating the grid-tie inverter and the electric power grid from each other using an isolation transformer; a method using a common mode choke coil that suppresses the common mode current; a method of bypassing the common mode current to an input side or the ground by a filter; a method of outputting voltages with reverse polarities to upper and lower arms while using two-level pulse width modulation (PWM) as a control method of the inverter; a method of composing, in the inverter, a bypass channel of which impedance becomes low for the high frequency common mode current, thereby preventing the leakage current and the high frequency noise from flowing outside; a method in which these methods are combined with one another; and the like.